The present invention relates to an optical compensatory sheet comprising an optically anisotropic layer formed of discotic liquid crystal molecules provided on a transparent substrate. The invention also relates to a liquid crystal display using the optical compensatory sheet.
A liquid crystal display comprises a liquid crystal cell, two polarizing elements arranged on each side of the liquid crystal cell, and one or two optical compensatory sheets (phase retarders) arranged between the liquid crystal cell and the polarizing element.
The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The rod-like liquid crystal molecules are provided between the substrates. The electrode layer has a function of applying a voltage to the rod-like liquid crystal molecule. Each of the substrates has an orientation layer, which has a function of aligning the rod-like liquid crystal molecule.
The optical compensatory sheet is used to remove color from an image displayed on the liquid crystal cell. The optical compensatory sheet can also be used to enlarge a viewing angle of a liquid crystal cell. A stretched birefringent film has usually been used as the optical compensatory sheet.
An optical compensatory sheet comprising an optically anisotropic layer on a transparent substrate has been proposed to be used in place of the stretched birefringent film. The optically anisotropic layer is formed by fixing aligned discotic liquid crystal molecules. The discotic liquid crystal molecules have various alignment forms. Accordingly, an optical compensatory sheet obtained by using the discotic liquid crystal molecule has a specific optical characteristic that cannot be obtained by the conventional stretched birefringent film. The optical compensatory sheet using the discotic liquid crystal molecule is described in Japanese Patent Provisional Publication No. 6(1994)-214116, U.S. Pat. Nos. 5,583,679, 5,646,703 and German Patent Publication No. 3,911,620A1.
Japanese Patent Provisional Publication No. 2(1990)-176625 discloses a liquid crystal display using a liquid crystal cell of a vertical alignment (VA) mode, in which rod-like liquid crystal molecules are essentially vertically aligned while not applying voltage to the cell, and are essentially horizontally aligned while applying voltage to the cell. The vertical alignment mode is characterized in a wide viewing angle and a fast response, compared with the conventional liquid crystal modes. A prototype of the liquid crystal display of a vertical alignment mode has been exhibited (Nikkei Microdevice (written in Japanese), No. 136, page 147, 1996).
U.S. Pat. Nos. 4,583,825 and 5,410,422 disclose a liquid crystal display of a bend alignment mode having a liquid crystal cell in which rod-like liquid crystal compounds are aligned symmetrically. The alignment of an upper liquid crystal compound is essentially antiparallel to the alignment of a lower liquid crystal compound. The liquid crystal cell of the bend alignment mode has a self-optical compensatory function because of the symmetrical alignment. Therefore, the bend alignment mode is also referred to as an optical compensatory bend (OCB) mode. The liquid crystal display of the bend alignment mode has an advantage of a rapid response.
The liquid crystal display of a vertical alignment mode or a bend alignment mode is characterized in a wide viewing angle and a rapid response compared with the conventional liquid crystal modes (TN mode, STN mode). However, a further improvement is necessary to be comparable with CRT.
It might be considered that an optical compensatory sheet is used to improve the liquid crystal display of a vertical alignment mode or a bend alignment mode in the same manner as in the conventional liquid crystal displays. However, the known optical compensatory sheets used in the conventional liquid crystal displays are not effective in the liquid crystal display of the vertical alignment mode or the bend alignment mode.
In the liquid crystal cell of the vertical alignment mode or the bend alignment mode, most of rod-like liquid crystal molecules are essentially vertically aligned. An optical compensatory sheet should have a specific optical anisotropy to optically compensate the optical anisotropy of the essentially vertically aligned rod-like liquid crystal molecules. The required specific optical anisotropy is a large negative optical anisotropy in which an optical axis is not inclined. A stretched birefringent film such as a biaxially stretched polycarbonate film can satisfy the requirement. However, the stretched birefringent film has a problem with dimensional stability and production cost.
An optical compensatory sheet comprising an optically anisotropic layer containing discotic liquid crystal molecules has been proposed to be used in place of the stretched birefringent film. However, the optically anisotropic layer of the known optical compensatory sheet does not have a large negative optical anisotropy in which an optical axis is not inclined.
The present inventors have tried to prepare an optical compensatory sheet having the required specific large negative optical anisotropy by using discotic liquid crystal molecules. However, it is very difficult to prepare a specific optically anisotropic layer in which an optical axis is essentially not inclined (less than 5xc2x0) where the layer is prepared according to prior art.
An object of the present invention is to provide an optical compensatory sheet having a large negative optical anisotropy in which an optical axis is essentially not incined.
Another object of the invention is to provide an optical compensatory sheet suitable for a liquid crystal display of a vertical alignment mode or a bend alignment mode.
A further object of the invention is to further improve a viewing angle of a liquid crystal display of a vertical alignment mode or a bend alignment mode.
The present invention provides an optical compensatory sheet comprising an optically anisotropic layer formed of discotic liquid crystal molecules provided on a transparent substrate, wherein the discotic liquid crystal molecules are horizontally aligned in the optically anisotropic layer, an average inclined angle between discotic planes of said discotic liquid crystal molecules and a surface of said transparent substrate being less than 5xc2x0, and said discotic liquid crystal molecules being fixed in the optically anisotropic layer while keeping the horizontal alignment.
The invention also provides a liquid crystal display comprising a liquid crystal cell of a vertical alignment mode, two polarizing elements and one or two optical compensatory sheets, each of said polarizing elements being arranged on each side of the liquid crystal cell, each of said optical compensatory sheets being arranged between the liquid crystal cell and each polarizing element, wherein the optical compensatory sheet comprises an optically anisotropic layer formed of discotic liquid crystal molecules provided on a transparent substrate, said discotic liquid crystal molecules being horizontally aligned in the optically anisotropic layer, and wherein an average inclined angle between discotic planes of the discotic liquid crystal molecules and a surface of the transparent substrate is less than 50xc2x0, said discotic liquid crystal molecules being fixed in the optically anisotropic layer while keeping the horizontal alignment.
The invention further provides a liquid crystal display comprising a liquid crystal cell of a bend alignment mode, two polarizing elements and one or two optical compensatory sheets, each of said polarizing elements being arranged on each side of the liquid crystal cell, each of said optical compensatory sheets being arranged between the liquid crystal cell and each polarizing element, wherein the optical compensatory sheet comprises an optically anisotropic layer formed of discotic liquid crystal molecules provided on a transparent substrate, said discotic liquid crystal molecules being horizontally aligned in the optically anisotropic layer, and wherein an average inclined angle between discotic planes of the discotic liquid crystal molecules and a surface of the transparent substrate is less than 5xc2x0, said discotic liquid crystal molecules being fixed in the optically anisotropic layer while keeping the horizontal alignment.
The present inventors have found that an optical compensatory sheet having a large negative optical anisotropy in which an optical axis is essentially not inclined can be obtained by adjusting an average inclined angle between discotic planes of discotic liquid crystal molecules and a surface of the transparent substrate to be less than 5xc2x0. However, it is difficult according prior art to align discotic liquid crystal molecules in such manner (homeotropic alignment). The present inventors have further studied the alignment of the discotic liquid crystal molecules, and have finally succeeded in aligning the discotic liquid crystal molecules having an average inclined angle of less than 5xc2x0. The homeotropic alignment of the discotic liquid crystal molecule is obtained by using a specific coating amount of a cellulose ester of a lower fatty acid, a fluorine containing surface active agent or 1,3,5-triazine compound.
Therefore, the optical compensatory sheet of the present invention has a large negative optical anisotropy in which an optical axis is essentially not inclined. The optical compensatory sheet of the invention is particularly effective in a liquid crystal display of a vertical alignment mode or a bend alignment mode, in which most of rod-like liquid crystal molecules are essentially vertically aligned.
For the reasons mentioned above, the excellent characteristics (a wide viewing angle and a rapid response) of a liquid crystal display of a vertical alignment mode or a bend alignment mode are further improved by using optical compensatory sheet of the present invention.